1. Field of the Invention
The present invention relates to a fuel cell system, a fuel cell power generation apparatus, and a method for operating the fuel cell power generation apparatus. More particularly, the present invention relates to a fuel cell power generation apparatus capable of adjusting the amount of fuel therein into a predefined range by controlling the operation point of the fuel cell, a method for operating the fuel cell power generation apparatus, and a fuel cell system including the fuel cell power generation apparatus.
2. Description of the Related Art
A fuel cell is a device that generates electricity from hydrogen and oxygen so as to obtain highly efficient power generation. A principal feature of a fuel cell is its capacity for direct power generation which does not undergo a stage of thermal energy or kinetic energy as in conventional power generation. This presents such advantages as high power generation efficiency despite the small scale setup, reduced emission of nitrogen compounds and the like, and environmental friendliness on account of minimal noise or vibration. A fuel cell is capable of efficiently utilizing chemical energy in its fuel and as such environmentally friendly. Fuel cells are therefore envisaged as an energy supply system for the twenty-first century and have gained attention as a promising power generation system that can be used in a variety of applications including space applications, automobiles, mobile devices, and large and small scale power generation. Serious technical efforts are being made to develop practical fuel cells.
Of various types of fuel cells, a polymer electrolyte fuel cell (PEFC) excels in its low operating temperature and high output density. Recently, direct methanol fuel cells (DMFC) are especially attracting the attention as a type of polymer electrolyte fuel cell. In a DMFC, methanol water solution as a fuel is not reformed and is directly supplied to the anode so that electricity is produced by an electrochemical reaction induced between the methanol water solution and oxygen. Discharged as reaction products resulting from the electrochemical reaction are carbon dioxide emitted from the anode and generated water emitted from the cathode. Methanol water solution has a higher energy density per unit volume than hydrogen. Moreover, it is suitable for storage and poses little danger of explosion. Accordingly, it is expected that methanol water solution will be used in power supplies for automobiles, mobile devices (cell phones, notebook personal computers, PDAs, MP3 players, digital cameras, electronic dictionaries and books) and the like.
When a DMFC generates power, the fuel is consumed. Therefore a DMFC system using a DMFC normally receives the supply of fuel from a fuel tank which is provided detachably on the DMFC system and which is filled with pure methanol or a high concentration methanol solution. And the arrangement is such that when the fuel tank becomes empty of the fuel, the fuel tank is replaced with a new one.
3. Related Art List
(1) Japanese Patent Application Laid-Open No. 2005-108811.
In a conventional fuel cell system, carbon dioxide discharged from the anode is separated from the fuel, which is also discharged from the anode, and released to the outside of the system. Accordingly, the arrangement is such that part of the fuel passage, especially the fuel storage area, is open to the outside. Yet, from this opening, the moisture and methanol in the fuel are also released together with carbon dioxide, and the amount of release (evaporation) also varies with the environmental conditions, such as temperature and humidity. As a result, there are cases where it is difficult to adjust the amount of fuel inside a fuel cell system to a constant level.